Doc: Add note to import QtQuick.Layouts when using SplitView

In order to use the Layout attached properties.

Task-number: QTBUG-31968

Change-Id: I00880b6a4ac91532db5da513d13757949fae64c2
Reviewed-by: Alberto Mardegan <mardy@users.sourceforge.net>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>